The present disclosure relates generally to communication management, and, more particularly, to methods for border cell handoff between third generation (3G) and second generation (2G) communication systems, and devices utilizing same.
One commonly used of cellular radiotelephone communication system is the Code Division Multiple Access (CDMA) system. In the CDMA system, the radio signals share the same frequency spectrum simultaneously, in contrast to Frequency Division Multiple Access (FDMA) or Time Division Multiple Access (TDMA) systems. One CDMA standard, known as a 2G standard, is designated as TIA/EIA-95-A/B. More recently, a 3G CDMA standard is proposed and designated as CDMA2000.
A mobile station communicates with a base station having the strongest available signal. In order to track available signals, the mobile station maintains a list of available base stations. Specifically, each base station in the CDMA system transmits an unmodulated pilot signal on a set of predetermined frequencies. A searcher unit in the mobile station detects the pilot signals and measures the strength thereof. The results from the searcher are reported to the current base station. The base station then instructs the mobile station to update a list of available base stations maintained by the mobile station. The list is sub-divided into an active set, a candidate set, and a neighbor set. The active set contains a list of base stations currently communicated with the mobile station. The candidate set is a list of base stations which may move into the active set. The neighbor set is a list of base stations which are being monitored, but less frequently.
As the mobile station moves and departs from a base station, the currently active signal sent from the base station weakens. The mobile station must access a new base station. The communication link must be handed off to the next base station. Two types of handoff, soft and hard, are employed in communication systems. In a soft handoff, a new link is established before an original link is terminated. In a hard handoff, a connection with a currently active base station (i.e., 3G) is terminated before a new connection with a new base station (i.e., 2G) is established.
Conventionally, in CDMA systems, the mobile station and base station perform handoff according to the pilot strength (Ec) and the total received power (Io) (Ec/Io). When the mobile station moves and the currently active pilot signal weakens, the mobile station must access a new base station and complete a 3G soft handoff. When the mobile station moves to a new 3G cell, the received pilot of the old cell will decrease and the received pilot of new cell will increase, as shown in FIG. 1. Curve OC indicates the Ec/Io value of the old cell, and curve NC indicates that of the new cell. At point T, a 3G soft handoff is performed. The best pilot transfers the pilot of the old cell to that of the new cell.
Currently, 2G systems provide the most worldwide coverage. In the near future, 3G systems will be deployed on top of 2G systems to provide more advanced services, as shown in FIG. 2. On border cells, however, 2G and 3G systems operate in different frequency bands. When the mobile station leaves the 3G coverage and accesses the area of 2G system, the pilot Ec/Io will decrease and no new 3G base stations can be accessed by the mobile station, as shown in FIG. 8. The dotted curves indicate the Ec/Io values of inner cells to be handed off. Therefore, the pilot Ec/Io will be maintained at a constant level for a significant range regardless of distance. As a result, Ec/Io is not a good criterion for determining handoff timing at border cells of the 2G and 3G systems, or an indication of quality degradation. Due to the differences in coverage between the 2G and 3G systems, it is desirable to provide a 3G to 2G handoff mechanism without losing the 3G system capacity.